The present invention relates to an optical transponder for secure optical communications and an optical quadrature-measuring instrument and in particular relates to quantum communications, quantum cryptography and optical communications.
Demand for secrecy in communications is a never ending theme from the ancient to the future, and in the network society in the recent years, development of cryptography has satisfied that demand. The security of public-key cryptosystem and the like which are currently popular is based on requirement for unrealistic time expenditure for decryption. However, computer technology is always progressing. Therefore, public-key cryptosystem and the like are not necessarily guaranteed on its security over the future. On the other hand, quantum cryptography that is actively being researched now is guaranteed by physical laws in security, and will not be deteriorated in security how far technology may be advanced, and realization thereof is being desired.
So far, various communication methods with techniques based on quantum mechanics have been proposed. However, the quantum state changes in an irreversible manner simply by influence of losses and external environments. Therefore, realization thereof is difficult in general. Under such circumstances, it has become apparent that utilization of antisqueezed component in a squeezed state being one of representative quantum states enables an application of quantum information technologies on a practical level (JP-A-2007-129386). The reason thereof is that the antisqueezed component can maintain its nature against the losses or amplification. Light including the antisqueezed component, that is, light with its non-isotropic shape of fluctuations on quadrature phase space is defined as antisqueezed light. Even if the antisqueezed light is amplified, its nature can be retained. Therefore, even if losses take place in transmission paths or respective kinds of components, it is possible to compensate the losses, and installation of a transponder station is feasible. However, all relays have to be carried out in the state of light. Although an all-optical network gradually increases so as to enable the relays to be carried out only with the optical processing, the relays only with the optical processing are naturally placed under various limiting conditions and a relay method of temporary conversion from optical signal to electric signal is important as well.
JP-A-2007-129386 and so forth intend to use quantum cryptography to, in general, prevent eavesdropping in a transmission path and naturally to enable a legitimate receiver to receive correct information. An electric circuit-based transponder is nothing else to receive and retransmit optical signals. Therefore, adoption of electric circuit-based transponder will temporarily settle a signal in the transponder station to expose itself to the risk of eavesdropping.